Ferroelectric liquid crystal device alignment

ABSTRACT

A ferroelectric liquid crystal device comprises a layer of a ferroelectric liquid crystal material contained between two cell walls carrying electrode structures and a surface alignment treatment. The surface alignment is provided by a profiled surface, e.g., a grating, on at least one cell wall. The grating may be a monograting or a bigrating, with a symmetric or asymmetric profile. Such a profiling enables surface tilt and alignment anchoring energy to be independently arranged to suit liquid crystal material and device type to give a required molecular arrangement and low device defect. The grating may be provided by interferography, photolithography, embossing, ruling, or carrier layer transfer. Alignment directions on the cell walls may be parallel or non-parallel. The surface tilt on both cell walls may be the same or different values. The cell walls may be relatively rigid, e.g., glass slides, or flexible, e.g., thin plastic which may have its inner face embossed to provide one or both a grating and a set of spacer pillars.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to ferroelectric liquid crystal device alignment.

2. Discussion of Prior Art

Liquid crystal display devices are well known. They typically comprise aliquid crystal cell formed by a thin layer of a liquid crystal materialheld between two glass walls. These walls carry transparent electrodeswhich apply an electric field across the liquid crystal layer to cause areorientation of the molecules of liquid crystal material. The liquidcrystal molecules in many displays adopt one of two states of moleculararrangement. Information is displayed by areas of liquid crystalmaterial in one state contrasting with areas in the other state. Oneknown display is formed as a matrix of pixels or display elementsproduced at the intersections between column electrodes on one wall androw electrodes on the other wall. The display is often addressed in amultiplex manner by applying voltages to successive row and columnelectrodes.

Liquid crystal materials are of three basic types, nematic, cholesteric,and smectic each having a distinctive molecular arrangement.

The present invention concerns chiral smectic liquid crystal materialsparticularly ferroelectric smectic liquid crystal materials. Devicesusing this material include the surface stabilised ferroelectric liquidcrystal (SSFLC) device. These devices can show bistability, ie theliquid crystal molecules, more correctly the molecular director, adoptone of two alignment states on switching by positive and negativevoltage pulses and remain in the switched state after removal of thevoltage. This behaviour depends, in part, upon the surface alignmentproperties. The switched states may be stabilised by the presence of anac bias which may be provided by the data (column) voltages in amultiplexed addressed device. Another type of ferroelectric liquidcrystal device (FELCD) is the electro-clinic device. SSFLC are describedfor example in N. A. Clark & S. T. Lagerwall, App Phys Letters 36(11)1980 pp 899-901, and U.S. Pat. No. 4,367,924. Yet another type is anantiferroelectric device (Ref: A. Fukuda et al. J later. Chem. (1994) 4,7, 997-1016).

Alignment of ferroelectric liquid crystals is often carried out usingrubbed polymer alignments, however this method has several limitations.These include dirt, electrical damage due to discharge between in-planeelectrodes and uniformity of pretilt and anchoring. Another knownalignment is that of oblique evaporation of eg SiO. This is difficult toemploy for large area displays.

It can be shown that if the alignment used introduces a splay in ahigher temperature cholesteric phase, then a predominantly (zig-zag)defect free alignment can be achieved in the ferroelectric phase. Thismay be due to the balance between surface pretilt and surface (zenithaland azimuthal) anchoring energies being able to bias the presence ofeither the C1 or C2 chevron at the operating temperature. In order to beable to optimise this balance for a given liquid crystal, ideally it isnecessary to be able to change surface pretilt and anchoring energiescontrollably and independently. This is difficult with rubbed polymersas all these properties are manifestations of the same physicalinteraction.

For some liquid crystal materials the alignment may be very sensitive tothis balance and so good uniformity of pretilt and anchoring energies isrequired over the surface, this is not always achievable with rubbedalignment techniques.

Furthermore in some FLC devices it is crucial to align the preferreddirections (defined in the cholesteric phase) accurately with respect toeach other, be these parallel or offset at some required angle. This isvery difficult with conventional alignments since rubbing leaves noeasily observable direction on the surface.

SUMMARY OF THE INVENTION

The above problems are solved according to this invention by the use ofgratings on the inner surface of liquid crystal cell walls.

According to this invention a liquid crystal device comprises a layer ofa ferro electric smectic liquid crystal material having at least asmectic A phase or a cholesteric phase at higher temperature containedbetween two cell walls each bearing electrode structures on their innersurface and each surface treated to align liquid crystal material at thesurface, characterised by a grating surface alignment on at least onecell wall.

Preferably the grating surface alignment gives independent control ofalignment anchoring strength and liquid crystal surface tilt anglewhereby a required splay of liquid crystal molecular director may beformed across the liquid crystal layer.

Larger azimuthal anchoring energy can be used for C2 ferroelectricswhile weaker anchoring can be used for C1 ferroelectrics orantiferroelectrics.

The grating surface may be a monograting or a bigrating, and may besymmetric or asymmetric (blazed) in any combination. The angle of thegratings between two cell walls may be substantially parallel, or nonparallel. A bigrating having at least one asymmetric (blazed) gratingcan result in both surface alignment and a surface pretilt. Two suchsurfaces may be arranged so that the liquid crystal molecules adopt asplayed configuration in a higher temperature long pitch cholestericphase.

The liquid crystal material may be a material having a ferroelectricliquid crystal phase at normal device operating temperatures,particularly those which show less ordered phases, eg smectic A andcholesteric phases, at higher temperatures.

In this context a bigrating surface is one that can be described by

    Y(x,y)=Y(x+mk.sub.x.y+nk.sub.y)

where Y is a function describing the surface amplitude, m and n areintegers and k_(x), k_(y) are periodicities. A single grating isinvariant in one of the principal directions x or y. A blazedmodulation, in say x-direction, is defined as a surface for which theredoes not exist a value of h such that

    Y(h-x)=Y(h+x)

for all values of x, this definition can similarly be applied to theother principal modulation (grating) direction (y)

An example of a non-blazed (symmetric) bigrating surface is the doublesinusoidal structure described by

    Y=a.sub.1 sink.sub.x x+a.sub.2 sink.sub.y y

if either a₁ or a₂ is set to zero then a single grating is recovered.

For monostable alignment to be achieved on a bigrating surface, in ahigher temperature cholesteric (or smectic A) phase then one gratingmodulation has to be energetically dominant over the other; ie a₁ ²k_(x) ³ >a₂ ² k_(y) ³. If a liquid crystal, possessing the phasesequence long pitch cholesteric--(Smectic A)--ferroelectric smectic, ispresent between two such grating surfaces, which are arranged with theirprinciple directions substantially parallel then heating into thecholesteric phase will give a monostable alignment which when cooledinto the ferroelectric smectic phase yields a well aligned liquidcrystal.

The bigrating may be a profiled layer of photopolymer formed by aninterferographic process; eg M. C. Hutley, Diffraction Gratings(Acedemic Press, London 1982) p95-125; or a photolithographic process egF Horn, Physics World, 33 (March 1993). Alternatively, the bigrating maybe formed by embossing; M. T. Gale, J Kane and K. Knop, J Appl PhotoEng, 4, 2, 41 (1978), or ruling; E. G. Loewen and R. S. Wiley, ProcSPIE, 815, 88 (1987), or by carrier layer transfer.

One or both cell walls may be formed of a relatively thick non flexiblematerial such as a glass, or one or both cells walls may be formed of aflexible material such as a thin layer of glass or a plastic material egpolypropylene. A plastic cell wall may be embossed on its inner surfaceto provide a grating. Additionally, the embossing may provide smallpillars (eg of 1-3 μm height and 5-50 μm or more width) for assisting incorrect spacing apart of the cell walls and also for a barrier to liquidcrystal material flow when the cell is flexed. Alternatively the pillarsmay be formed by the material of the alignment layers.

BRIEF DESCRIPTION OF THE DRAWINGS

One form of the invention will now be described, bit way of exampleonly, with reference to the accompanying drawings of which:-

FIG. 1 is a plan view of a matrix multiplex addressed liquid crystaldisplay:

FIG. 2 is a cross section of the display of FIG. 1;

FIG. 3 is a diagrammatic view of apparatus for producing gratingsurfaces of a cell wall:

FIG. 4 is a diagrammatic view of alternative apparatus for producinggrating aligning surfaces.

FIG. 5-12 show various configuration of gratings on cell walls.

DETAILED DISCUSSION OF PREFERRED EMBODIMENTS

The display of FIGS. 1, 2 comprises a liquid crystal cell 1 formed by alayer 2 of ferroelectric liquid crystal material contained between glasswalls 3, 4. The material 2 has a less ordered higher temperature phase,eg smectic C, smectic A, long pitch cholesteric with increasingtemperatures. A long pitch cholesteric can be defined as a cholestericpitch greater than layer thickness. A spacer ring 5 maintains the wallstypically 2 μm apart.

Additionally numerous 2 μm diameter beads may be dispersed in the liquidcrystal material to maintain an accurate wall spacing. Strip like rowelectrodes 6 eg of SnO2 or ITO are formed on one wall 4 and similarcolumn electrodes 7 formed on the other wall 3. With m-row and n-columnelectrodes this forms an m.n matrix of addressable elements or pixels.Each pixel is formed be the intersection of a row and column electrode.

A row driver 8 supplies voltage to each row electrode 6. Similarly acolumn driver 9 supplies voltages to each column electrode 7. Control ofapplied voltages is from a control logic 10 which receives power from avoltage source 11 and timing from a clock 12.

Either side of the cell 1 are polarisers 13, 13'. Conventional surfacestabilised ferroelectric bistable devices switch between two stablestates having alignment either side of a surface alignment directionfavoured by the rubbing directions. For devices of the present inventionboth cell walls 3, 4 have bigrating alignment and switch between twostable states having alignment either side of a principle gratingdirection. The polarisers 13, 13' are arranged with their polarisationaxis crossed with respect to one another with the axis of one polariserparallel to one of the two (switched) stable state alignment directions.

A partly reflecting mirror 16 may be arranged behind the cell 1 togetherwith a light source 15. These allow the display to be seen in reflectionand it from behind in dull ambient lighting. For a transmission device,the mirror may be omitted.

Prior to assembly the cell of FIGS. 1, 2 at least one cell wall issurface treated to provide a bigrating; the other wall may have either abigrating or a monograting or a conventional eg rubbing alignmenttreatment. Apparatus for producing this bigrating is shown in FIG. 3.

As shown in FIG. 3 light 20 from an argon ion laser 21 is focused by afirst lens 22 onto a fixed first diffuser 23 and a rotating second 24diffuser. A second lens 25 recollimates the now expanded laser beam ontoa semi aluminised beamsplitter 26. Light is reflected from thebeamsplitter 26 onto a first mirror 27 and thence onto a substrate 28supported in a holder 29. Light transmitted through the beam splitter 26is reflected off a second mirror 30 and a third mirror 31 onto thesubstrate. Thus the substrate 28 receives two beams 20a, 20b which setsup a stationary fringe pattern. The pitch of the fringe pattern dependsupon the angle between the two beams 20a, 20b coming from the first andthird mirrors 27, 31.

A sinsoidal (symmetric) bigrating may be produced by the apparatus ofFIG. 3 as follows:

EXAMPLE 1

A piece of ITO coated glass 28 to form a cell wall was cleaned inacetone and isopropanol and Was then spin coated with a photopolyimide(Nissan RN901) at 4000 rpm for 20 seconds to give a coating thickness of1.2 μm. Softbaking was then carried out at 80° C. for 30 minutes. Thesample 28 was then exposed to an interference pattern of light generatedfrom the argon ion laser 21 (wavelength of 457.9 nm) as shown in FIG. 3.

The sample 28 was given a 90 second exposure at a power density of 1.5mW/cm². A second exposure also of 90 seconds duration was then carriedout after the sample 28 had been removed from the holder 29, rotated by90° and replaced. Development was then carried out by a 60 secondimmersion in microposit MF319 developer followed by a 30 second rinse indeionised water. Finally the photopolyimide was crossed linked by a 60minute bake at 170° C. followed by a 30 minute bake at 350° C. In thiscase the resulting sample contained a surface relief bigrating in whichthe two principle modulations were at 90° to each other. However it maybe advantageous for particular applications if the modulations were atless than 90° to each other, eg 45°.

The technique above can be used to generate single or double non-blazedgratings. One simple technique for generating a bigrating where one ofthe principal modulation directions is blazed (nonsymmetric) is shown inFIG. 4.

A substrate 35 carrying a thin indium tin oxide electrode 36 is coatedwith a photopolymer 37 such as Ciba Geigy 343. Spaced above the polymer37 is a mask 38 formed be a chrome pattern 39 on a glass slide 40. Thechrome pattern is that of a bigrating photolithography mask. A mercurylamp 41 is arranged to illuminate the mask 38 at a non normal angle tothe mask surface. The off axis illumination, in a plane defined by themask normal and one of the mask principal directions, ensures that ablazed profile is obtained (after development in QZ3301 and rinsing inQS3312) in one direction but a non-blazed profile is obtained in theother principal direction. Off-axis illumination that is not in a planedefined by the mask normal and one of the principal directions will givea bigrating that is blazed in both principal directions.

The following FIGS. 5-12 show different arrangements of gratings in acell.

FIG. 5 shows diagrammatically symmetric (non-blazed) and asymmetric(blazed) profiles together with an indication of double arrows forsymmetric and single arrows for blazed direction.

FIG. 6 shows two single grating surfaces aligned substantially parallel(in the same or opposite directions). The shaded ellipses show the LCdirector alignment obtained in the higher temperature cholesteric phaseat each surface. The director alignment is in the same direction withzero surface pretilt in eg a cholesteric phase.

FIG. 7 shows two grating surfaces, the top wall having a non blazedgrating (the vertical arrow) and optionally a second non blazed grating(horizontal arrow with broken line) subservient to the other grating.This results in a director alignment shown to be horizontal at the topwall surface and with zero surface tilt. The bigrating on the bottomsurface is similar. These two bigratings give substantially non-twisted,monostable alignment in the higher temperature cholesteric phase.

FIG. 8 shows a bigrating at the top surface giving alignment and zerosurface tilt. The bottom surface has a symmetric grating and a blazedgrating. The symmetric grating is dominant so that the liquid crystaldirector aligns horizontally, but with a surface tilt provided by theshape of the blazed grating.

FIG. 9 shows two similar bigratings, a symmetric and an asymmetricgrating on both cell walls; the symmetric gratings are dominant. Theseprovide both alignment direction and surface tilt giving a splayeddirector configuration in a cholesteric phase.

FIG. 10 is similar to FIG. 9 except that the blaze direction on thelower surface is in the opposited direction to that on the top surface.These give a substantially non-twisted, monostable alignment in a highertemperature cholesteric phase.

FIG. 11a shows two single or bi-grating surfaces azimuthally offset withrespect to each other such that the cell would align a cholestericliquid crystal in a monostable twisted state. The offset angle can beadjusted to stabilise or destabilise half-splayed states in aferroelectric liquid crystal in a chevron layer geometry.

FIG. 11b shows two bigrating surfaces each of which have two preferredalignment states for a cholesteric in contact with the surface. Bothgratings in each bigrating are of equal anchoring energy which resultsin the two preferred alignment direction. The bigratings are arrangedoffset to each other to give a substantially uniform monostablealignment in a cholesteric phase. The angle between the bistable statesat each surface can be adjusted to stabilise or destabilise half-splayedstates in a ferroelectric liquid crystal in a chevron layer geometry.

FIG. 12 shows two bigrating surfaces, each with both principaldirections blazed to give substantially untwisted alignment in a highertemperature cholesteric phase. Both bigratings provide alignment andsurface tilt. The example shown has two of the blaze directions paralleland in the same direction, while the other (dominant) two are paralleland in opposite directions; this allows the adjustment of the stabilityof the chevron states and also the stability of half-splayed states. Anadvantage of this arrangement is that the anchoring energy vs alignmentdirection can be made asymmetric in opposite directions on the two cellwalls.

We claim:
 1. A liquid crystal device comprising a layer of aferroelectric smectic liquid crystal material having at least a smecticA phase or a cholesteric phase at higher temperatures contained betweentwo cell walls each bearing electrode structures on their inner surfaceand at least one cell wall having a surface treatment on said innersurface to align liquid crystal material at the surface,wherein saidsurface treatment comprises two monogratings, said monogratings on atleast one wall where one of said monogratings is energetically dominantover the other.
 2. The device of claim 1 wherein the grating surface isa monograting.
 3. The device of claim 1 wherein the grating surface is abigrating.
 4. The device of claim 1 wherein at least one grating surfaceis symmetric.
 5. The device of claim 1 wherein at least one gratingsurface is asymmetric.
 6. The device of claim 1 wherein surfacealignment directions provided on both cell walls by the gratings aresubstantially parallel.
 7. The device of claim 1 wherein surfacealignment directions provided on both cell walls are at a non zero angleto one another.
 8. The device of claim 1 wherein the liquid crystalmaterial is a ferroelectric material having the following phases withdecreasing temperatures:--isotropic--cholesteric--smectic A--chiralsmectic or isotropic--cholesteric--chiral smectic.
 9. The device ofclaim 1 wherein both cell walls have a bigrating surface, and the twogratings are arranged to provide a splayed molecular arrangementthroughout the liquid crystal layer in a higher temperature phase. 10.The device of claim 1 wherein both cell walls have a bigrating surface,the bigrating comprising a symmetric grating and an asymmetric gratingwith the symmetric grating being energetically dominant over theasymmetric grating to give a monostable alignment and a surfacemolecular pretilt in the smectic A or cholesteric phase of the liquidcrystal material, the monostable alignment and pretilt directions beingarranged to provide a splayed molecular arrangement throughout theliquid crystal layer in the higher temperature phase.
 11. The device ofclaim 9 wherein the bigrating is a layer formed by interferography,pholtolithography, embossing, ruling, or carrier transfer.
 12. Thedevice of claim 1 wherein the cell walls are formed of a glass material.13. The device of claim 1 wherein the cell walls are formed of aflexible plastic material.
 14. The device of claim 1 wherein spacerpillars are formed on one or both cell walls.
 15. The device of claim 1wherein spacer pillars are formed by a material forming the grating onone or both cell walls.
 16. The device of claim 1, wherein each of saidtwo monogratings is on a separate inner surface of said two cell walls.17. The device of claim 1, wherein both of said two monogratings are ona single inner surface of one of said two cell walls.
 18. The device ofclaim 17, wherein said two monogratings on a single cell wall comprise abigrating.